The major objectives of this project are: (1) to study the effects of immunosuppression on the pathogenesis of experimental viral infections particularly those which involve the central nervous system (CNS) and, (2) to use the immunosuppressed, virus-infected animal as a recipient of adoptively transferred lymphoid cells or serum from syngeneic, virus-immune donors in an attempt to assess the relative contributions of the cell-mediated and humoral immune responses to the outcome of infection. Immunosuppression is accomplished primarily through the use of cyclophosphamide, anti-lymphoid sera or thymectomy. Currently under study are West Nile virus, which produces a direct cytolytic effect on susceptible neuronal elements, and lymphocytic choriomeningitis virus, a non-cytopathic agent which produces immune- mediated CNS disease.